Surface seismic and vertical seismic profiling (VSP) methods often provide a good image of the overall geological structure of a hydrocarbon reservoir, but these methods are typically unable to describe smaller-scale reservoir features. Sonic and other well logging methods provide excellent resolution of bedding layers intersecting the borehole, but these measurements typically reflect conditions only in the immediate vicinity of the borehole. Sonic imaging was created to bridge the gap in spatial resolution between these seismic and well logging methods.
The assignee of the present application, Schlumberger, has developed a wireline sonic imaging tool (referred to as the Borehole Acoustic Reflection Survey (BARS) tool) that allows reservoir features such as reflectors and fractures to be imaged. Additional information regarding sonic imaging in general and the BARS tool in particular may be found in the following documents, each of which is incorporated herein by reference:
Japanese Patent Office laid open publication number 10-62555, published Mar. 6, 1998, entitled “Method and Apparatus for Borehole Acoustic Reflection Logging”;
Watanabe, S., Fujii, K., and Mikada, H.: “Reflector imaging using Borehole Acoustic Reflection Survey,” Proc., the 4th Well Logging Symposium of Japan, SPWLA-J, Makuhari, Japan, September 1998, paper Q;
Chang, C., Hoyle, D., Coates, R., Kane, M. R., Dodds, K., Esmersoy, C. and Foreman, J. 1998. Localized maps of the subsurface, Oilfield Review, Spring Issue;
Esmersoy, C., Chang, C., Kane, M. R., Coates, R. T., Tichelaar, B. and Quint, E., 1998. Acoustic imaging of reservoir structures from a horizontal well: The Leading Edge, Soc. Expl. Geophys., 17, 940-946;
Coates, R. T., Kane, M. R., Chang, C., Esmersoy, C., Fukuhara, M., Yamamoto, H., 2000. Single well sonic imaging: High-definition reservoir cross-sections from horizontal wells, SPE-CIM Int'l Conf. Horizontal Well Technology; and
Yamamoto, H., Watanabe, S., Koelman, J. M. V., Geel, J., Brie, A., Fujii, K., Coates, R., 2000. Borehole acoustic refection survey experiments in horizontal wells for accurate well positioning, SPE-CIM Int'l Conf. Horizontal Well Technology.
While significant successes in sonic imaging have been obtained, data acquired for sonic imaging purposes typically contain many types of arrivals in addition to the desired reflected arrivals, such as tool-borne noise and borehole-borne noise (direct compressional and shear headwaves and tube waves, for instance). Effectively attenuating this noise is an important step in properly processing this sonic imaging data.
Another issue important in the processing and interpretation of sonic imaging data is determining whether the reflectors are above, below, or to the side of the wellbore. Various methods of attempting to determine reflector positions are known in the art. Typically, these methods require assumptions to be made regarding the geometry of the subsurface reflectors or the geometry of the subsurface reflectors with respect to the desired borehole trajectory. A method that is capable of determining the position of the reflectors independently of these types of assumptions is clearly desirable.
Accordingly, an object of the present invention is to provide an improved method of processing sonic data acquired in a borehole in which tool-borne and borehole-borne arrivals may be attenuated and/or the azimuthal positions of reflectors with respect to the borehole may be determined.
A further object of certain embodiments of the present invention is to provide an improved method of using processed sonic data for geosteering purposes while drilling.